Resettable switching device

ABSTRACT

A resettable switching device, e.g. a relay, comprises a fixed contact ( 18 ) and a movable contact ( 28 ). A solenoid ( 12 ) is fixed relative to the fixed contact and a ferromagnetic plunger ( 20 ) carries the movable contact. A spring ( 24 ) biases the plunger away from the fixed contact so the device is normally open. When the device is set a further ferromagnetic element, e.g. a plunger ( 22 ), holds the first plunger ( 20 ) in a closed-contact position by magnetic attraction against the action of the spring ( 24 ). When a predetermined current condition exists in the solenoid the magnetic attraction between the element and plunger is reduced below the level necessary to hold the plunger so that the movable contact disengages the fixed contact.

The present invention relates to a resettable switching device forclosing, holding closed, and opening a set of electrical contacts, andmay be used in applications such as residual current devices, circuitbreakers, relays and similar applications.

According to the present invention there is provided a resettableswitching device comprising at least one fixed contact and at least onemovable contact carried by a movable contact carrier, the movablecontact carrier including a first ferromagnetic element, a solenoidfixed relative to the fixed contact, a resilient biasing means biasingthe contact carrier towards a first position wherein the movable contactdoes not engage the fixed contact, and a second ferromagnetic elementfor drawing the first element to and holding it in a second position bymagnetic attraction against the action of the resilient bias, themovable contact engaging the fixed contact in the second position of thefirst element, wherein when a predetermined current condition exists inthe solenoid the magnetic attraction between the second element and thefirst element is reduced below the level necessary to hold the firstelement in the second position so that the first element is released bythe second element and moves towards the first position under the actionof the resilient bias and the movable contact disengages the fixedcontact.

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a first embodiment of the inventionwith the contacts open;

FIG. 2 shows the first embodiment with the contacts closed;

FIG. 3 is a schematic diagram of a second embodiment of the inventionwith the contacts open;

FIG. 4 shows the second embodiment with the contacts closed;

FIG. 5 is a schematic diagram of a third embodiment of the inventionwith the contacts open; and

FIG. 6 shows the third embodiment with the contacts closed.

FIG. 7 is a schematic diagram of a fourth embodiment of the inventionwith the contacts open.

FIG. 7A is a side view of the moving contact carrier of FIG. 7 with thecontacts open.

FIG. 8 is a view similar to FIG. 7 of the fourth embodiment with thereset button pushed upwardly to initiate closure of the contacts.

FIG. 9 is a view similar to FIG. 7 showing the fourth embodiment withthe contacts closed.

FIG. 9A is a side view of the moving contact carrier of FIG. 7 with thecontacts closed.

FIG. 10 is a schematic diagram of a fifth embodiment of the inventionwith the contacts open.

In the drawings the same reference numerals have been used for the sameor equivalent components.

Referring first to FIGS. 1 and 2, the device is mounted on a printedcircuit board (PCB) 10 or other item of electrical equipment onto or inwhich the device is to be incorporated. A fixed solenoid 12, comprisinga bobbin 14 and winding 16, is mounted on the PCB 10 and on either sidethereof a respective pair of fixed electrical contacts 18 (so-calledrivet contacts) are also mounted on the PCB. A first ferromagneticplunger 20 is slidably mounted in the top end of the solenoid and asecond ferromagnetic plunger 22 is slidably mounted in the bottom end ofthe solenoid (terms of orientation such as “top” and “bottom” refer tothe orientation of the device as seen in the drawings and does not limitits orientation in use). Each plunger is resiliently biased by arespective compression spring 24, 26. The springs bias the plungers 20,22 mutually away from one another so that each tends to be pushed, byits respective spring, in a direction out of the solenoid 12. The firstplunger 20 carries movable electrical bridging contacts 28 on a contactcarrier 30 mechanically coupled to the plunger. The second plunger 22has a manual reset button 27.

FIG. 1 shows the situation with no or negligible current flowing in thewinding 16. In that case the plungers 20, 22 are held apart by theirrespective springs 24, 26 with a substantial air gap 32 between themand, in particular, the plunger 20 is held in a first position whereinthe bridging contacts 28 are held out of engagement with the fixedcontacts 18.

When a current flows through the winding 16 an electromagnetic force isgenerated which will induce a magnetic attraction between the twoplungers 20, 22. In use of the device, the magnitude of this current ischosen to be sufficiently low as to avoid automatic closing of the airgap between the plungers, although above a pre-determined thresholddiscussed below. Thus, although each plunger may move slightly towardsthe other against its respective biasing spring, the magnetic attractionbetween the two plungers is not sufficient to significantly reduce theair gap 32.

However, if the plunger 22 is manually pushed upwardly into the bobbin14, against the bias of the spring 26, so as to sufficiently reduce theair gap 32 between the two plungers, the magnetic attraction inducedbetween the two plungers will increase to the point where the plunger 22magnetically entrains the plunger 20. The springs 24, 26 are designedsuch that the spring 26 tending to push the entrained plungers downwardsis sufficiently strong to overcome the spring 24 tending to push themupwards, so that if the plunger 22 is now released it moves downwardlyonce again towards its initial (FIG. 1) position. This will draw theplunger 20 downwards and further into the body of the solenoid 12 withthe result that the mechanically coupled moving contact carrier 30 willalso be drawn downwards. The downward travel of the plunger 20 will stopwhen the moving bridging contacts 28 come to rest (under pressure) onthe fixed contacts 18, thereby closing the normally open contacts.

The plunger 20 will be held in this second position as long as themagnitude of the current flowing through the winding 16 is greater thanthe predetermined threshold referred to above, which is that currentmagnitude sufficient to induce a magnetic attraction between theentrained plungers greater than the force of the springs 24, 26 tendingto separate them. This is referred to as the steady state magneticforce. However, if the magnitude of the current through the winding 16is reduced below the predetermined threshold the steady state magneticforce will in turn be reduced and the force of the springs 24, 26 willcause the two plungers to separate and thereby allow each plunger torevert to its initial (FIG. 1) position and the bridging contacts 28disengage the fixed contacts 18.

The embodiment of FIGS. 1 and 2 is known as an electrically latchingmechanism because the mechanism can only be latched when a current ofsufficient magnitude flows through the solenoid winding 16. A secondembodiment shown in FIGS. 3 and 4 provides for a mechanically latchingmechanism which can be latched in the absence of current flow throughthe winding. In the embodiment of FIGS. 3 and 4, the plunger 20 isreplaced by a plunger 120 having substantially the same dimensions asthe plunger 20 but which is a permanent magnet. In all other respectsthe structure of the embodiment of FIGS. 3 and 4 is the same as that ofFIGS. 1 and 2.

In the initial open state, FIG. 3, no or negligible current flowsthrough the winding 16. The magnetic attraction between the plungers 120and 22, generated by the permanent magnetism of the plunger 120, isinsufficient to draw the two plungers together (i.e. to significantlyreduce the air gap 32 between the two plungers). However, when theplunger 22 is manually pushed into the bobbin 14 the air gap 32 issufficiently reduced that plunger 22 magnetically entrains plunger 120.When the plunger 22 is released it moves towards its first (FIG. 3)position, drawing plunger 120 and the movable contact carrier 30 in thesame direction. The entrained plungers 22, 120 and the contact carrier30 will come to rest when the movable contacts 28 engage the fixedcontacts 18. The device is now in the closed state (FIG. 4).

The magnetic force generated by the permanent magnet (plunger 120) underthis condition is referred to as the steady state magnetic force and issufficiently strong to overcome the combined force of the springs 24, 26tending to separate them, and ensures reliable operation throughadequate contact pressure at rated load current.

Any current flow though the winding 16 will result in the establishmentof an electromagnetic field within the solenoid. Dependent on thepolarity of the current, this magnetic field will be in the samedirection or in the opposite direction to that of the permanent magnet.If the electromagnetic field is in the opposite direction it will reducethe steady state magnetic force holding the plungers 22, 120 together.By increasing the current magnitude through the winding 16 from anegligible level, a state will eventually be reached where the net forceof magnetic attraction between the plungers is no longer strong enoughto hold them together against the force of the springs 24, 26 tending toseparate them, at which point the plungers will spring apart and revertto their initial (FIG. 3) positions. The magnetic force generated by thecurrent through the winding need only to be of sufficient strength toweaken the net magnetic force to a level where separation of theplungers is assured. This means that the current level through the coilcan be optimised to achieve the desired opening of the contacts withoutincurring the problems of power dissipation or component stresses thatcould arise from the use of larger current levels.

In the embodiments of FIGS. 1 to 4, the two plungers are of uniformsection with parts of each plunger extending outside the solenoid body.Due to the air gap between them, the solenoid initially exerts anattracting force on each plunger, attempting to draw each into the bodyof the solenoid and minimise the air gap. The steady stateelectromagnetic force is insufficient of its own to close the air gap.However, as the air gap between the two plungers is closed as described,there will initially be a directional force applied to both plungerstrying to draw them into the solenoid body. However, once the twoplungers become entrained, this directional force will cease due to theuniformity of the two plungers and the fact that parts of the plungerswill still extend outside the body of the solenoid even when thecontacts are closed. The net downward force will then be entirely due tothe difference between the forces of the springs 24 and 26, theelectromagnetic force being used solely to keep the two plungersentrained. This arrangement allows the contact pressure to be easilyquantified and controlled by virtue of the two springs which aretherefore substantially the sole determinant of the pressure between thefixed and movable contacts when the contacts are closed.

However, the electromagnetic force can also be used to contributetowards or to determine contact pressure if desired. This can beachieved by modification of the plunger designs so as to maintain adirectional force on them after entrainment. For example, the plungermaterials could be different, or plunger 20/120 could be tapered suchthat the upper part is of a larger cross sectional area than the lowerpart. Due to the larger cross sectional area of the upper part of theplunger, the solenoid will exert a downward pulling force on plunger20/120 at all times. Under this arrangement the spring 26 can bedesigned to have a force equal to or less than that of spring 24 suchthat the electromagnetic force on the entrained plungers issubstantially the sole determinant of the pressure between the fixed andmovable contacts when the contacts are closed. Such arrangements toachieve directional force are well known in the solenoid and relayindustries. The downward force contributed by the solenoid could be usedto manipulate the operation of the device in terms of operatingcharacteristics, component characteristics and costs, etc.

The first and second embodiments described above involve manualoperation of the device to achieve the closed state. However, the devicecan also be configured in a third embodiment (FIGS. 5 and 6) to providefor automatic closing of the contacts. The construction of this thirdembodiments differs from that of FIGS. 1 and 2 only in that the plunger22 and associated spring 26 are replaced by a fixed ferromagnetic polepiece 122.

In operation of the device a continuous steady state current flowsthrough the winding 16, but this current is not of a magnitude to inducea magnetic attraction between the pole piece and the plunger 20 ofsufficient strength to draw the plunger 20 to the pole piece 122 againstthe force of the spring 24. The device contacts 18, 28 therefore remainopen (FIG. 5). To close the contacts, a pulse of current ofsubstantially higher magnitude is caused to flow through the winding fora short duration. This pulse of current is referred to as the pull-incurrent. This results in a substantially stronger magnetic field whichis sufficient to attract the plunger 20 down into the solenoid body andto substantially close the air gap 32 between the plunger and polepiece, the downward movement of the plunger 20 resulting in closure ofthe normally open contacts (FIG. 6). With the air gap so reduced oreliminated, the current magnitude can be reduced to the initial steadystate value and the force of magnetic attraction between the plunger andthe pole piece will remain sufficient to hold the plunger in thissecond, closed-contacts position. This steady state current is referredto as the holding current. However, if the holding current is reducedbelow a predetermined threshold, the magnetic attraction between thepole piece and plunger will become insufficient to hold the plunger inthe second position against the force of the spring 24, and the plungerwill revert to its first position, thereby opening the contacts.

Automatic re-closing of the contacts will occur when the pull-in currentis reapplied and the holding current restored. To ensure automaticopening and to prevent unwanted re-closing of the contacts, arrangementscan be made with suitable circuitry to ensure that the flow of theholding current and/or the surge current pulse is sufficiently reducedor disabled following the opening action. A reset means can be providedto overcome the disabling means and restore the automatic closingfunction.

FIGS. 7 to 9A show another embodiment of the invention. This embodimentcomprises a solenoid 12 including a bobbin 14 within which is fitted amovable ferromagnetic plunger 22 having a reset button 27, the plunger22 and reset button 27 being biased into a first position (FIG. 7) by acompression spring 26. The bobbin 14, which has a coil (not shown) woundon it, is fitted to a printed circuit board 10 on which are also fittedtwo fixed contacts 118. The embodiment further comprises an invertedgenerally U-shaped moving contact carrier 30 and is fitted with twoelectrical contacts 128. The contact carrier 30 is resiliently biasedaway from the PCB 10 by, in this embodiment, a spring arm 124 so as tomaintain the moving contacts 128 normally out of contact with the fixedcontacts 118. The moving contact carrier 30 contains a compartment 120into which is situated a permanent magnet 122.

When the reset button 27 is pressed towards the bobbin 14, it reducesthe air gap 32 between the top of the plunger 22 and the permanentmagnet 122, and when the air gap is sufficiently reduced the permanentmagnet is drawn towards the plunger and magnetically couples with it,bringing the moving contact carrier 30 from its first position to anintermediate position as shown in FIG. 8. When the reset button 27 isreleased, the plunger 22 is returned towards its first position by theforce of the reset spring 26 which is greater than the force of thespring 124 tending to hold the moving contact carrier 30 in the openposition. Throughout this action, the permanent magnet 122 remainsmagnetically coupled to the plunger 22, and hence the plunger 22,contact carrier 30 and moving contacts 128 all move in train towards thefirst position of the plunger 22 when the reset button is released.

As the plunger moves towards its first position, FIG. 9, the movingcontacts 128 come into contact with the fixed contacts 118, preventingany significant further travel of the plunger 22 towards its initialposition. At this stage, the contacts 118/128 are closed and thecontacts pressure is a function of the force exerted by the reset spring26.

When a current flows through the coil of the bobbin, it will generate anelectromagnetic field with North and South poles. Dependent on thedirection of flow of the current, the electromagnetic pole produced atthe top of the plunger 22 will be the same as or opposite to that of thepermanent magnet 122, causing the plunger and magnet to further attracteach other or to repel each other. By arranging for the current flow toproduce opposing magnetic fields at the interface of the plunger andpermanent magnet, the net magnetic attraction between the two parts willbe reduced. When this magnetic holding force is sufficiently reduced, byan increase in the current above a certain threshold, the opening forceof the biasing means 124 acting on the moving contact carrier 30 willcause the moving contacts 128 to separate from the fixed contacts 118 tobring the device to the open position, FIG. 7. Thus automatic opening isprovided by the flow of a current of appropriate magnitude and directionthrough the coil.

A feature of the above embodiment is that when the contacts 118/128 arein the closed position, there is still a certain amount of travelavailable to enable the reset button 27 and plunger 22 to return to theinitial position of FIG. 1. Thus, the reset button has two distinctpositions, the contacts open position and the contacts closed position.The difference in these two positions may be used to indicate thecontact open and closed states.

Furthermore, if an additional downward (as seen in FIG. 9) force ofsufficient magnitude is applied to the reset button 27 when the contactsare in the closed position, the reset button and plunger will be drawnto their first position. Such a force may be applied manually by pullingthe reset button towards its first position. Given that the movingcontact carrier 30 will not be able to move further in the direction ofthe PCB 10, due to the engagement of the contacts 118/128, an increasingair gap will be opened between the permanent magnet 122 and plunger 22,with a resultant weakening of the magnetic holding force. The design canbe arranged to ensure that when the reset button is drawn to its initialposition, the bias 124 acting on the moving contact carrier 30 issufficient to move the latter automatically to its initial contacts-openposition (FIG. 7). Thus, this embodiment is provided with manual openingmeans in addition to the automatic opening means.

The embodiment of FIG. 7 does not require any electrical energy toenable the circuit breaker to be closed, but does require electricalenergy to automatically open the circuit breaker. The embodiment of FIG.10 is an electrically latching version of the embodiment of FIG. 7. Inthe embodiment of FIG. 10, a non-ferromagnetic spacer 200 has beenplaced on the underside of the permanent magnet 122. This spacer has theeffect of ensuring that a minimum air gap is maintained between theplunger 22 and the permanent magnet 122 when the plunger is presented tothe permanent magnet. Due to the air gap, the magnetic coupling betweenthe plunger and the permanent magnet will be relatively weak and as aresult closing of the contacts will not be possible by use of thepermanent magnet alone. To facilitate closing of the circuit breaker, acurrent is passed through the coil which generates an electromagneticfield which produces a polarity at the top of the plunger 22 so as toresult in an increased magnetic coupling force. When this current issufficiently increased, the permanent magnet 122 will be magneticallyentrained with the plunger 22 and the moving contact carrier 30 can bebrought to the second position under the force of the reset spring 26 soas to ensure closing of the fixed and moving contacts 118/128. When thecurrent through the coil is reduced below a certain threshold, themagnetic force of the permanent magnet 122 will not be strong enough tomaintain entrainment with the plunger 22, and the moving contacts 128will move automatically to the open position. Thus, in the embodiment ofFIG. 10, the presence of a current of sufficient magnitude and directionfacilitates manual closing of the contacts, and reduction of themagnitude of this current results in automatic opening of the contacts.

The basic functionality of both embodiments of FIGS. 7 and 10 can beachieved as shown herein and in other ways without departing from theprinciples of the invention. For example, in the embodiment of FIG. 10,weakening of the permanent magnet attracting force could be achieved bythe use of a weaker magnet, or by reducing the length of the plunger orby reducing the cross sectional area of the plunger, etc. The mechanismcould be fitted on to any suitable medium other than a printed circuitboard. An opening spring could be fitted between the bobbin and themoving contact carrier to obviate the need for spring biased movingcontact arm, etc. A flag indicator may be fitted to the moving contactcarrier or the moving contacts to indicate the contact open and closedstates, etc.

Enhancements can be made to the embodiments described above, such asprovision of a ferromagnetic frame to improve the magnetic performanceof the device, or to provide means to indicate the open and closedstates of the contacts, etc., without detracting from the basicprinciple of operation.

The invention is not limited to the embodiments described herein whichmay be modified or varied without departing from the scope of theinvention.

1. A resettable switching device comprising a solenoid for mounting withits axis substantially perpendicular to a circuit board, a movablecontact closure member having a pair of arms which extend along oppositesides of the solenoid, each arm being arranged to bring a movablecontact into engagement with at least one respective contact fixed tothe circuit board adjacent to the solenoid, a first ferromagneticelement, a resilient biasing means for biasing the contact closuremember towards a first position wherein the movable contacts do notengage the fixed contacts, and a second ferromagnetic element fordrawing the first element to and holding it in a second position bymagnetic attraction against the action of the resilient bias, themovable contact engaging the fixed contact in the second position of thefirst element, wherein when a predetermined current condition exists inthe solenoid the magnetic attraction between the second element and thefirst element is reduced below the level necessary to hold the firstelement in the second position so that the first element is released bythe second element and moves towards the first position under the actionof the resilient bias and the movable contact disengages the fixedcontact, wherein one end of the solenoid is fixedly mountable to thecircuit board, the movable contact closure member is disposed at theopposite end of the solenoid to the said one end and the movable contactclosure member includes the first ferromagnetic element:
 2. A resettableswitching device as claimed in claim 1, wherein the first ferromagneticelement is a permanent magnet, and wherein the second ferromagneticelement is movable in the solenoid, against a further resilient biasingmeans, towards the permanent magnet to magnetically entrain the latterand upon release of the second element to draw the contact, closuremember, under the action of the further resilient biasing means, to thesecond position.
 3. A resettable switching device as claimed in claim 2,wherein the permanent magnet is fitted within the movable contactclosure member.
 4. A resettable switching device as claimed in claim 2,wherein the permanent magnet and second ferromagnetic element are heldtogether against the first and second resilient biasing means tending toseparate them by the force of attraction between them, the predeterminedcurrent condition being the presence of a solenoid current of sufficientmagnitude and direction to induce a magnetic field in opposition to thatof the permanent magnet so that the force of attraction between thepermanent magnet and second ferromagnetic element becomes less than theforce of the resilient biasing means tending to separate them.
 5. Aresettable switching device as claimed in claim 2, wherein one of thepermanent magnet and second ferromagnetic element has anon-ferromagnetic spacer which maintains a minimum separation betweenthem such that the second ferromagnetic element can only entrain thepermanent magnet by the additional magnetic attraction produced by asolenoid current above a predetermined threshold, the predeterminedcurrent condition being the reduction of the solenoid current below thethreshold.
 6. A resettable switching device as claimed in claim 1,wherein the second element comprises a plunger slidable in the solenoid,the first and second elements being biased by respective resilientbiasing means mutually away from one another, and wherein the plunger ismovable in the solenoid against its resilient bias to magneticallyentrain the first element.
 7. A resettable switching device as claimedin claim 6, wherein the resilient bias acting on the plunger issufficiently strong to overcome the resilient bias on the first elementthat upon release of the plunger the latter draws the first elementinto, and holds the first element at, the said second position in theabsence of the said predetermined current condition.
 8. A resettableswitching device as claimed in claim 7, wherein the difference in theforces exerted by the respective resilient biasing means issubstantially the sole determinant of the pressure between the fixed andmovable contacts when the first element is in the second position.
 9. Aresettable switching device as claimed in claim 6, wherein the first andsecond elements are held together against the respective resilientbiasing means tending to separate them by magnetic attraction induced bya solenoid current above a predetermined threshold, the predeterminedcurrent condition being the reduction of the solenoid current below thethreshold.
 10. A resettable switching device as claimed in claim 9,wherein the electromagnetic force on the entrained elements issubstantially the sole determinant of the pressure between the fixed andmovable contacts when the first element is in the second position.
 11. Aresettable switching device as claimed in claim 6, wherein the first andsecond elements are held together against the respective resilientbiasing means tending to separate them by permanent magnetism of atleast one of the elements, the predetermined current condition being thepresence of a solenoid current of sufficient magnitude and direction toinduce a magnetic field in opposition to that of the permanent magnet sothat the force of attraction between the elements becomes less than theforce of the resilient biasing means tending to separate them.
 12. Aresettable switching device as claimed in claim 6, wherein the first andsecond elements are respective plungers entering the solenoid fromopposite ends.
 13. A resettable switching device as claimed in claim 1,wherein the second ferromagnetic element comprises a fixed pole piece,the first element being drawn towards the pole piece against itsresilient bias by magnetic attraction induced by a sufficiently highsolenoid current and being held in its second position by the pole pieceby magnetic attraction induced by a solenoid current above apredetermined threshold which is less than the said sufficiently highcurrent, the predetermined current condition being the reduction of thesolenoid current below the threshold.